High levels of serum cholesterol have been repeatedly correlated with an increased incidence of coronary heart disease in epidemiological surveys. Studies where diet has been used to reduce the serum cholesterol level, in an attempt to determine whether lowering cholesterol levels in "high risk" persons is beneficial, offer inconclusive evidence. This multi-institutional trial seeks to test whether marked lipid reduction can cause retardation, arrest or reversal of the atherosclerotic process in individuals who have had an atherosclerotic myocardial infarction. The method of achieving lipid reduction will be the partial ileal bypass operation, supplemented by dietary advice. This operation is the most effective means of reducing lipid levels available today, is safe and has a lasting effect. In addition, this mode of intervention has the statistical desirable quality of being obligatory in its actions. Patients referred by private physicians will be screened for eligibility. Eligible patients, who are willing to participate, being fully informed as to the nature of the trial, will be assigned by a random process to either surgery or non-surgery. The goal is to find 1,000 eligible and willing patients; half of these will be operated upon and the other half will form a control group. Certain clinical metabolic and radiologic studies will be conducted in association with this trial.